1. Technical Field of the Invention
The present invention is a system for storage and retrieval of digital data. More particularly, the present invention involves processing digital recorded material with joint signal detection techniques.
2. Background Art
Digital data storage devices, such as computer drives and portable tapes, compact discs and floppy diskettes, are recording components in many electronic devices, and typically provide mechanisms for storing and retrieving large amounts of data quickly and reliably. Digital recorders, as used herein, refer to the many embodiments employed for storing digital information in a variety of digital systems and a multitude of applications. The most common form of digital recorder is a rotating radial magnetic disk. Other digital recorders include but are not limited to optical disks and magnetic tape systems, including linear devices.
The prior art disk drive system is well known in the art. A data storage disk, such as floppy disks, hard disks, and cubical disks as well as linear and multi-level disks all function in a similar fashion. The common radial disk contains a number of concentric data cylinders that contains several data sectors. The sectors are located on an upper side of the disk and additional sectors may be located on a lower side or in multiple layers within the disk. The disk is accessed by a head element mounted on an arm that is secured to the drive. The disk is accessed via photoemitters/photoreceptors for optical systems and with magnetic read/write elements as discussed herein for magnetic systems wherein various accompanying electronic circuits are familiar to those of skill in the art.
Using disk drives as an example, the disk is typically subdivided into one or more partitions by using a partition table that is located on the disk. A wide variety of partitions file systems as discussed in the prior art are not necessary for a proper understanding of the present invention. A given sector on the disk is usually identified by specifying a head, a cylinder, and a sector within the cylinder. A triplet specifying the head number, cylinder number, and sector number in this manner is known as a physical sector address. Alternatively, a given sector may be identified by a logical sector address, which is a single number rather than a triplet of numbers.
In more specific detail, for a data storage device, such as a magnetic disc drive, the recording medium is typically divided into a plurality of generally parallel data tracks. The data is stored and retrieved by a transducer or head element that is positioned over a desired data track by an actuator arm. The head element can be a combined read/write head or separated into a read head and a write head in close proximity.
The actuator arm typically moves the head across the data tracks under the control of a closed-loop servo system based on servo data stored on the disc surface within dedicated servo fields. The servo fields can be interleaved with data sectors on the disc surface or on a separate disc surface that is dedicated to storing servo information. As the head passes over the servo fields, it generates a readback servo signal that identifies the location of the head relative to the centerline of the desired track. Based on this location, the servo system rotates the actuator arm to adjust the head""s position so that it moves to the desired position.
There are several prior art types of servo field patterns, such as a null-type servo pattern, a split-burst amplitude servo pattern, and a phase type servo pattern. A null type servo pattern includes at least two fields which are written at a known phase relation to one another. The first field is a phase or sync field which is used to lock the phase and frequency of the read channel to the phase and frequency of the read signal. The second field is a position error field that is used to identify the location of the head with respect to the track centerline.
In a typical prior art embodiment, as the head passes over the position error field, the amplitude and phase of the read signal indicates the magnitude and direction of the head offset with respect to the track centerline. The position error field has a null-type magnetization pattern such that when the head is directly straddling the track centerline, the amplitude of the readback signal is ideally zero. As the head moves away from the desired track centerline, the amplitude of the read signal increases. When the head is half-way between the desired track centerline and the centerline of the adjacent track, the read signal has a maximum amplitude. The magnetization pattern on one side of the centerline is written 180 degrees out of phase with the magnetization pattern on the other side of the centerline, and the phase of the read signal indicates the direction of the head position error.
To control the servo system, a single position error value is normally generated for each pass over the position error field. Typically, the magnitude of the position error value indicates the distance of the head from the track centerline, and the sign of the position error value indicates the direction of the head""s displacement. The position error values are typically created by demodulating the read signal associated with the position error field. In a synchronous process, the exact phase of the read signal from the position error field is known from the phase field""s read signal because the phase field is written on the storage medium at a known and fixed phase relation to the position error field. A phase-locked loop (PLL) is typically used to acquire the phase of the phase field, and this phase information is used for demodulating the position error field.
Processing of the read signal is generally demodulated by generating a demodulating signal, such as a square wave, having the same phase and frequency as a fundamental component of the read signal and then, with analog techniques, multiplying the read signal by the demodulating signal. The product is integrated over a time window that corresponds to the middle cycles of the position error field. The result is a position error value for the head with respect to a desired position on the storage medium within that servo pattern. This process essentially identifies the amplitude and phase of the read signal at a specific frequency point. The sign of the position error value indicates which direction the head is located with respect to the desired location.
The most common application for digital recorders is the computer disk drive. All sizes of computers including portable laptops, personal computers and mainframes include a digital recording system. Typically the recording device is a magnetic disk drive, but other devices such as optical disks and tape systems are also commonly used. Besides computers, other digital systems also use digital recorders, for example, digital video cameras write data to a digital recorder in the form magnetic tape, magnetic hard disks or optical disks.
Magnetic disc drives, because of their greater speeds, have become the medium of choice for storing frequently accessed data such as application programs and user data which is being created or frequently modified. Conventional magnetic disk drive storage systems have been commonly used and are well known in the art. These storage systems typically use a flying magnetic read/write head, either combined or separate read head and write head, to record and retrieve data from a layer of magnetic recording material on the surface of a rotating recording disk. The capacity of such a storage system is a function of the number of closely spaced concentric tracks on the recording disk that may be reliably accessed by the read/write head. Some parts of the recording disk surface area may be used for purposes other than data storage.
For example, means for assuring the proper selection of a particular track by the read/write head are required for reliable data storage and retrieval. The read/write head is typically aligned and kept centered over a particular track as the recording disk rotates, to prevent accidental over-writing of data stored in neighboring tracks and to minimize inter-track interference. Some systems use nonmagnetic guard rings between discrete tracks on the recording disk to help keep the head from skipping off-track. Gain control references may be placed at different locations on the recording disk to calibrate the electronic amplifiers used to reliably read back data signals. Time delay elements are also sometimes used to allow the magnetic read/write head to demagnetize after recording data to prevent unintentional over-writing of subsequently accessed locations. The prior art designs take up some of the available recording disk surface area, and thus reduce overall system capacity.
In a magnetic hard disk drive, data bits are stored as transitions between ferromagnetic domains, the absence of transitions, or some combination thereof, indicating a one and a zero respectively. When the read/write head floats over the spinning drive, the transition shows up as a pulsed waveform while the absence of a transition shows up as a flat waveform. By synchronizing and detecting pulses, the read head decodes the ones and zeros on the disk. Each symbol on a track is a xe2x80x98bitxe2x80x99 and generally takes the form of a Lorentzian pulse. Magnetic media tends to be two state storage devices due to the physics.
The xe2x80x98bitsxe2x80x99 as referenced herein refer to any of the schemes that allow for a xe2x80x981xe2x80x99 and a xe2x80x980xe2x80x99 to be detectable by the read element and subsequent processing. For example, the data bits interpreted as xe2x80x981xe2x80x99s and xe2x80x980xe2x80x99s may be a magnetic transitions such as polarized and not polarized; polarized and reversed polarized; a transition between two states; or absence of transitions. It is presumed that there will continue to be improvements into the manner in which bits can be written and read, all of which are within the scope of the invention.
Signal processing is used to some extent to optimize the storage on the in-track direction. However, no similar technique currently applies to adjacent track interference, which is known to severely limit disk drive performance. There have been attempts to increase the density of disks in order to have more narrow magnetic domains in the cross-track direction. While the write technology has advanced to allow denser writing, the read technology has been limiting factor in data density.
The importance and significance of signal processing is detailed in the article from IEEE Signal Processing Magazine, July 1998, entitled xe2x80x9cThe Role of SP in Data-Storagexe2x80x9d by Jackyun Moon. The problems related to intersymbol interference (ISI) are described along with the prior art processing techniques involving sequence detectors and symbol-by-symbol detectors. The symbol detectors are more likely to be effected by ISI, whereas the sequence detectors make symbol decision based on the observation of signals over many symbol intervals. Examples of sequence detectors include maximum-likelihood sequence detectors (MLSD), finite or fixed delay tree search detectors (FTDS) and partial-response maximum likelihood (PRML) techniques.
However, the prior art is replete with the attempts and problems with decreasing the spacing in order to put more digital information in a smaller space on the recording medium. While the write elements are technically capable of writing in a smaller area, the read element has limitations that restrict the size of the storage cells and the spacing. An example reference would be Roh, Lee and Moon, xe2x80x9cSingle-Head/Single-Track Detection in Interfering Tracksxe2x80x9d, IEEE Transactions on Magnetics vol 38 page 1830 from July 2002. This reference and many others discuss how interference from adjacent tracks due to head misalignment or other effects that tend to become the dominant source of read errors.
There has been considerable research in gigabit-density recording, including an article by Tsang, Chen and Yogi, which discusses the xe2x80x9cGigabit research in Gigabit-Density Magnetic Recordingxe2x80x9d, Proceedings of the IEEE, Vol. 81, No. 9, September 1993. This article illustrates the need for advanced processing that can take advantage of the high density disk recording of the data recorded as transitions that relate to the abrupt Magnetization changes on the tracks of the disk.
There are a variety of factors that limit the read/write capabilities of storage media, including various types of noise, inter-track interference, intersymbol interference, and non-linear distortion. Numerous equalization and coding schemes have evolved to provide more accurate determinations and permit greater density storage. One type of noise source for magnetic disk storage is the result of the recording head positioning error. This off-track or inter-track interference (ITI) can be modeled and reduce the associated errors. There are types of noise that are random, such as transition noise that occurs due to random variations in the geometry of magnetic transitions. There are also types of nonlinearities that have a repeatable characteristic and these distortions can be modeled and eliminated. One manner for describing the nonlinear distortions uses Volterra functional series which constructs the nonlinear portion of the signal as the sun of the outputs of nonlinear kernels. Also, the read head is sensitive to magnetic domains and adjacent tracks, and even if perfectly aligned, tracks must be spaced far enough apart to allow distinctions for processing. One reference for this noise model would be T. Oenning and J. Moon, xe2x80x9cModeling the Lorentzian Magnetic Recording Channel with Transition Noisexe2x80x9d from IEEE Transactions on Magnetics volume 37 page 583 (January 2001).
Magnetic recording devices, such as magnetic disks and tapes, use heads to read and write information to and from a magnetic surface. In a typical rotating storage system, data is stored on magnetic disks in a series of concentric tracks. These tracks are accessed by a read/write head that detects variations in the magnetic orientation of the disk surface. In most embodiments the read/write head moves back and forth radially on the disk under control of a head-positioning servo mechanism so that it can be selectively positioned over a specific track. Once the head is aligned over a track, the servo mechanism causes the head to trace a path that follows the center line of the selected track. Tracks as discussed herein refer to any segments parallel to relative motion of the sensor.
The recording head induces a magnetic field with sufficient amplitude to record on the magnetic material of the storage device to a sufficient depth. The magnitude and direction of the magnetic flux is modulated to encode information into the magnetic surface of the storage device. A pattern of external and internal fields are created as the head and recording surface are moved relative to each other. The polarity transitions are then readable as transitions in the magnetic flux at the recording surface. In read mode, as the magnetic storage surface moves across the gap in the head, the magnetic field of the storage surface is detected, and a voltage is induced in the head proportional to the rate of change of the flux. The read channel then processes the analog voltage signal to obtain the digital data.
Various types of indexing marks and alignment indicia are also recorded on the recording disk surface for precise position reference and tracking adjustment of the read/write head. These marks and indicia are often recorded in servo sectors, which are angularly-spaced reserved portions of the recording disk surface that extend out approximately radially from the recording disk centers. Track addresses are sometimes recorded in servo sectors. Angular synchronization signals that determine the circumferential location of the magnetic head may also be recorded in servo sectors. Normal and quadrature servo blocks are often recorded in servo sectors for generation of position error signals that are used to keep the read/write head aligned. Servo sectors use recording disk surface area that could otherwise be used for data storage, however, so servo sector information should be stored as efficiently as possible.
A typical prior art read process commences as the analog read signal originates from the read head which is then amplified in the preamplifier and then provided to a filter for the removal of high-frequency noise components. The filtered signal is then provided to a phase-locked loop clock circuit and delay line. The delay line provides the delayed signal for the analog-to-digital converter (ADC) where the signal is digitized. The digitized signal is passed through an equalizer to obtain a more desirable waveform, and the result is provided to a decoder. The decoder implements a decoding algorithm to generate the digital data signal. The analog-to-digital converter and decoder are clocked by a clock signal generated in a phase-locked loop clock circuit.
With respect to in-track processing, it was recognized early on that the single symbol bit processing was not satisfactory in dealing with ISI and noisy signals, and partial response maximum likelihood (PRML) processing provided certain benefits. In the PRML channel characterized by the polynomial (1xe2x88x92D)(1+D), a notch filter is generally used because the frequency response requires a sharp cutoff and the frequency spectrum is very different from that of the channel response in magnetic recording. A variation of PRML is extended partial response maximum likelihood (EPRML) that obviates the need for the notch filter. However, the Viterbi type computations for maximum likelihood detection become a limiting factor in terms of decoding speed and cost. Furthermore, both PRML and EPRML channels are very sensitive to mis-equalization or changes in signal shape due, for example, to component tolerances and to nonlinearities of the magnetic recording process such as caused by pulse asymmetry and the crowding of write transitions on the media. Moreover, the problems associated with cross-track interference still remain.
Early magnetic storage devices used analog peak detection to process incoming read signals. However, as recording density increased, the analog peak detection scheme became unreliable because of the large amount of inter-symbol interference (ISI) between adjacent pulses. The partial response maximum likelihood (PRML) channel has been used to increase the recording density, but the PRML method requires equalization of the read signal, and the code scheme is incompatible with the run-length limited (RLL) code. In addition, the required number of magnetic flux transitions per inch is much higher. Therefore, the magnetic non-linearity problem is more severe for the PRML system, and could even render it unusable at high recording densities.
Run-length limited (RLL) codes are used to place an upper bound on the number of data clock cycles occurring between signal transitions, and the clock recovery is based on the occurrence of these transitions. RLL codes ensure that sufficient transitions occur for the clock recovery circuit to maintain the correct timing phase and frequency. In an NRZI format, each 1 is represented by a transition, and each 0 is represented by the lack of a transition, and the RLL code is sufficient for clock recovery purposes. Also, by maintaining the minimum of one 0 between consecutive 1""s, transitions are separated so as to be differentiable from one another.
A signal processing method that uses RLL codes to improve the detection margin at high recording densities is described in U.S. Pat. No. 4,945,538. In U.S. Pat. No. 4,945,538, sample values of an analog signal corresponding to binary data are coded with a RLL code. The coded analog input signal is converted to a sequence of digital sample values and the signal is equalized to correspond to a predetermined analog shape. A sequence detection algorithm is used to decode the digital sample values into the coded binary data.
A different approach to increase the capacity and speed of optical data-storage systems uses multilevel optical recording systems. The term multilevel refers to more than two levels of data recorded on the medium. The density of data recorded on an optical recording medium is increased by modulating the reflectivity of the optical recording medium into more than two states. However, at high data densities, light reflected from one mark will tend to interfere with light reflected from adjacent marks, causing intersymbol interference (ISI). The effect of the ISI is greater when the marks are closer together.
Optical data disc readers primarily have involved analog filtering of the read signal to equalize the frequency response of the system in order to predict how much contrast an optical imaging system will generate when scanning different spatial frequencies. Digital equalization is generally superior to analog equalization, as discussed in U.S. Pat. No. 5,818,806. And, a method for providing digital equalization filters for multilevel data-storage systems and a compensating scheme for intersymbol interference is described in U.S. Pat. No. 6,377,529.
Phase-change technology has been around since 1995, and the PD drive combines an optical disk drive capable of handling high capacity disks along with a multi-speed CD-ROM drive. It uses purely optical technology, and relies on the use of a laser to write new data with just a single pass of the read/write head. In the PD system, the active layer is made of a material with reversible properties, and a high-power laser heats the portion of the active layer where data is to be recorded. The heated area cools rapidly, forming an amorphous spot of low reflectivity. A low-powered laser beam detects the difference between these spots and the more reflective, untouched, crystalline areas, thus identifying a binary xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d. By reheating a spot, recrystallisation occurs, resulting in a return to its original highly reflective state. Laser temperature alone changes the active layer to crystalline or amorphous according to the data required, in a single pass.
Compact discs (CD""s) are examples of digitally recorded data, and typically ascribe to the ISO 9660 standard. CD""s originated from audio applications, so the amount of information a CD can hold is measured in minutes:seconds:sectors. Each second contains 75 sectors, each of which can hold 2048 bytes (2 kilobytes) of Mode 1 user data. Recordable CD""s presently are available in a variety of sizes, namely 21- (80 mm diameter), 63-, and 74-minute sizes (both 120 mm diameter), which can contain the following amounts of data in the CD-ROM format:
21 minxc3x97(60 sec)xc3x97(75 sectors)xc3x97(2 kbytes)=189,000 kilobytes=184 megabytes
63 minxc3x97(60 sec)xc3x97(75 sectors)xc3x97(2 kbytes)=567,000 kilobytes=553 megabytes
74 minxc3x97(60 sec)xc3x97(75 sectors)xc3x97(2 kbytes)=660,000 kilobytes=650 megabytes
Factory-recorded CD""s generally hold 74 minutes of audio or 650 MB of data. There are several overhead fields that must be deducted when calculating the total amount of data that you can fit on a CD: Session Lead-In and Lead-Out. The first lead-in and lead-out on a disc are not usually taken into consideration when calculating space available on disc, and they are considered to be outside the usable disc area.
Files on CD do not occupy a space exactly equal to their original size, because the minimum recordable unit on a compact disc is the logical block. Logical block size depends upon the size of the drive and is calculated by an intrinsic formula. The larger the drive, the larger the logical block size, hence the more space a given file will consume.
The more portable recording mediums such as the traditional floppy disks are slowly relenting to other mediums with greater capacity. With state of the art hard disks measured in gigabytes, and with multimedia and graphics file sizes often measured in tens of megabytes, a capacity of 100 MB to 150 MB is required whether moving a few files between systems, archiving or backing up individual files or directories, and sending files by electronic mail.
Magnetic tape data storage devices, also referred to as tape drives, have been used in the computer industry for years for the storage of large amounts of data. Tape drives have achieved preeminence as storage devices for portable storage and long-term and data backup purposes.
With respect to portable storage, devices such as Iomega""s Zip drive provide use a technology developed by Iomega that draws the flexible disk upward towards the read/write head rather than moving the head toward the medium. Another portable scheme is LS-120, later termed SuperDisk, which resembles 1.44 MB 3.5 in disk, but uses a refinement of the floptical technology to deliver much greater capacity and speed. Named after the LS-120 laser servo technology it employs, an LS-120 disk has optical reference tracks on its surface that are both written and read by a laser system. These servo tracks are much narrower and can be laid closer together on the disk, wherein an LS-120 disk has a track density of 2,490 tracks per inch (tpi) compared with 135 tpi on a standard 1.44 MB floppy. Another option for portable storage is Sony""s HiFD drive, having a capacity of over 200 MB per disk. Compatibility with conventional 1.44 MB floppy disks is provided by equipping the HiFD with a dual-head mechanism. When reading 1.44 MB floppy disks, a conventional floppy-disk head is used and comes into direct contact with the media surface. The separate HiFD head works more like a hard disk, gliding over the surface of the disk without touching it.
With respect to hard drives, most hard drives are multi-GB, whether removeable or fixed. For removable drives, there are various options including the Iomega Jaz drive, SyQuest""s 1.5 GB SyJet and 1 GB SparQ. While generally similar to a hard disk the Jaz drive for example employs twin platters that sit in a cartridge protected by a dust-proof shutter which springs open on insertion to provide access to read/write heads. The Castlewood ORB was the first universal storage system to be built using magnetoresistive (MR) head technology, making them very different from other removable media drives that use older hard drive technology based on thin film inductive heads. MR hard drive technology permits a much larger concentration of data on the storage medium.
Optical storage using the blue laser is of considerable interest because the smaller wavelength of the drive""s laser light limits the size of the pit that can be read from the disc, thus having a narrow beam that can read smaller dots. The DVD Forum""s Steering Committee are promoting a proposed formatxe2x80x94dubbed xe2x80x9cBlu-ray Discxe2x80x9dxe2x80x94that is capable of providing storage capacities of up to 27 GB and 50 GB on single-layer and dual-layer discs respectively. The driving force behind such huge capacities is the emergence of multimedia applications in relation to both high-quality digital video and audio into the PC mainstream, coupled with the emergence of high-definition TV (HDTV), which is debuting in terrestrial broadcast systems.
Regardless of the technology underlying the portable disks, DVD""s, CD""s or tapes, or fixed/removable hard drives, including magneto optical, and rotating magnetic media the overall trend is to write/read in a compact format and optimize the space required with a stable and robust system.
Newer magneto-optical technology offers many improvements over conventional magnetic technology, particularly in terms of increased capacity. Magneto-optical storage systems also record data onto a recording material coated onto the surfaces of one or more rotating recording disks, but via different means than conventional drives. The recording material undergoes a sharp increase in magnetic susceptibility when heated beyond its Curie point, the temperature at which the magnetic properties of the recording material change from ferromagnetic to paramagnetic. A localized magnetic domain is created by heating a region of the recording material and then applying a magnetic field of a desired orientation to the heated region. When the recording material cools, the localized magnetic domain retains its magnetic orientation and again becomes far less susceptible to applied magnetic fields.
An optical fiber may guide an intense beam of focused laser light to heat a localized magnetic domain to be recorded or overwritten. The data stored in a particular localized magnetic domain may also be read back nondestructively by such a combined laser and optical fiber system. A low-powered, linearly polarized laser beam focused on a particular localized magnetic domain will be reflected with a Kerr rotation of the angle of polarization determined by the magnetic orientation of the localized magnetic domain. The pattern of polarization rotations read back as the low-powered laser beam moves across the recording surface thus represents the pattern of magnetic orientations previously written onto the recording surface. The overall reflectivity of a localized magnetic domain may also be determined via measurement of the relative amplitude of the reflected laser beam.
Magneto-optical storage systems quickly and reliably locate and align to any particular storage location on the recording disk, as with existing storage systems. A scheme for accomplishing these goals that takes advantage of the unique properties of a magneto-optical storage system is needed. An efficient system for encoding servo sector information is therefore important for maximizing the amount of remaining disk surface area available for data storage and retrieval.
The technology of tape drives has evolved from large, expensive open reel machines to the current generations of cassette tape drives, which store large amounts of data in convenient self-contained cassettes. Historically, open reel tape drives recorded data on parallel data tracks which extend along the length of the tape, and utilized fixed data recording/retrieval heads, i.e., one dedicated read/write head for each data track.
The actual recording and recovery of data on the tape medium is accomplished by a gap in the read/write head, and is in the form of magnetic flux reversals formed in the magnetic coating on the tape. To maximize the sharpness of the flux reversals and increase the amplitude of the read data pulses induced in the head during subsequent read operation, the length of the head gaps is aligned as precisely as possible with the direction of tape motion past the heads.
Historically, in order to ensure the integrity of data written on the tape, the tape drives included multi-gap heads, with one gap employed to write data and another gap, immediately trailing the write gap along the direction of tape motion, used as a read gap which could perform a read/verify operation on the data just recorded. If the tape drive was intended to record/recover data with the tape moving in both directions, an additional write or read gap was needed.
Several cassette-type tape drive formats are industry standards, including the format referred to as the QIC, or quarter inch cassette. In QIC format tape drives, data is recorded on a plurality of data tracks which extend parallel with the length of the tape as was typical in open reel type tape drives, but employ only a single recording/playback head which is controllably movable to each of the data tracks. A commonly used mechanism for controlling the movement of the head from track to track employs a worm gear driven by a stepper motor, with the pitch of the worm gear and the radial precision of the stepper motor determining the accuracy of head movement, including the repeatability of multiple head movements to any one given track.
One of the major factors controlling the overall storage capacity of tape storage devices is referred to as track density, typically defined in data tracks per inch of tape width, or how closely the data tracks are spaced. The greater the track density, the greater number of tracks that can be recorded on a given width of tape and the greater the overall cassette data capacity. A known factor limiting track density is referred to as adjacent track interference, which is the corruption or loss of data brought about when data on a given track is written at a location touching or even overlapping the previously recorded data on an adjacent data track. In such a situation, the amplitude of the readback signal can be reduced, and there is a limit to the amount of readback signal reduction which can be tolerated and beyond which data can be corrupted or lost completely.
Another factor controlling the ability of the tape drive to recover previously recorded data is a characteristic of the tape drive referred to as head azimuth, or simply azimuth, which is a measurement of the alignment between the longitudinal direction of the data tracks and the gap of the read/write head.
In the specifications defining the QIC tape drive and tape cassettes, one of the major planar surfaces of the cassette, called the cassette base plate, contains features which define a datum referred to as the tape cassette -B- plane. The tape cassette -B- plane is used, in conjunction with mating features on the tape drive which comprise a tape drive -B- plane, to define a mating surface between the tape cassette and the tape drive, and thus a base datum for defining the locations of both tape cassette and tape drive components and features along an axis normal to the common -B- plane. Because the data tracks extend along the length of the tape, the length of the head gap which accomplishes the recording and retrieval of data on the data tracks is nominally parallel with the length of the data track and thus also nominally parallel to the -B- plane. It is known, however, that small deviations from this nominally parallel relationship are introduced by component and manufacturing tolerances. It is this geometric relationship between the length of the head gap and the -B- plane which is referred to as azimuth. When the length of the head gap is parallel to the -B- plane, or, in other words, when the width of the gap is perpendicular to the -B- plane, azimuth is considered to be zero, with deviations from parallel in a first direction being referred to as positive azimuth and deviations in the opposite direction being referred to as negative azimuth. Non-zero azimuths are typically measured in units of rotation, such as minutes.
Tape drives used for recording video images have made use of this knowledge for several years to reduce intertrack interference and maximize the amount of storage on a given area of tape surface.
U.S. Pat. Nos. 5,307,217 and 5,371,638, for instance, (hereinafter the ""217 and ""638 patents, respectively) disclose apparatus and methods directed to recording data at opposite azimuth angles on adjacent data tracks in order to minimize intertrack interference, and thus maximize data capacity on tape media. There are, however, several differences in both the type of tape drive in which the disclosed method and apparatus are employed and in the specific apparatus which implements the recording of data at opposite azimuth on adjacent data tracks.
Thus, there is a growing demand for increased storage capacity and increased speed. What is needed therefore, is a method and apparatus that allows for more efficient storage and retrieval of digital data. More particularly, such an invention should allow digital recording with joint signal detection techniques for devices where efficient use of a storage medium is desirable in terms of optimizing device size, access speed and power consumption. Applications can include disk drives, tape systems, storage implementations for digital cameras, PDA""s, and any related devices employing digital storage media regardless of whether the technology is magnetic, optical or future storage technology.
The invention is devised in the light of the problems of the prior art described herein. Accordingly it is a general object of the present invention to provide a novel and useful digital recording implementation that uses multi-user detection techniques that can solve the problems described herein. The digital recording devices of the present invention include, but are not limited to various forms of tapes, disks, disc drives and virtually any device that has recorded data on a medium that is extracted by a sensing element.
Accordingly, it is an object of the present invention to provide a digital recording system consisting of a storage medium, a data writing element, a data reading element capable of reading closely spaced interfering bits written on the storage medium, a read signal processing element and appropriate servo mechanisms and controllers for engaging the read and write elements with the storage medium. Joint signal detection is known to separate interfering digital signals in the same channel, provided that the interfering signals have sufficient power relative to the noise floor in the channel. The present invention applies this technique to mitigate adjacent track interference by simultaneously demodulating signals arising from multiple closely spaced tracks underneath the reading element. It has been contemplated and within the scope of the invention to include read-only devices that would eliminate the writing elements and simplify the invention.
Another object of the present invention is to provide digital recorders that can recover data from media packed so densely that adjacent bits written to the medium interfere when accessed by the read mechanism.
Yet another object of the present invention is to provide digital recorders that can simultaneously demodulate multiple digital bits with a single read sensor.
A further object of the present invention to provide data storage scheme that provide parallel access to related parts of data files stored on the recorder to increase bandwidth.
A still further object of the present invention is to provide digital recorders with higher data storage density than recorders with single channel read sensors.
Another object of the invention is to provide digital recorders with faster data access to large files stored in the medium by exploiting parallel demodulation capability of the joint detection read sensor. Another problem with digital storage access is the rate at which data can be read from the storage medium. In applications where large files must be read, such as images and video, it is desirable to increase the access rate. The ability to simultaneously demodulate multiple tracks would in conjunction with a buffering mechanism, enable faster data transfer from the disk. A further object accomplished by the present system is that with the closer and denser tracks, there is an improved and faster track-to-track seek time.
Another object of the invention is to provide a means for reducing the power consumed by digital recorders by using the parallel demodulation capability of the joint detection read sensor. This implementation also reduces the mechanical speed of the storage medium and further reduces the digital clock frequency of the symbol sampling and synchronization circuits along with all other signal conditioning circuits commonly used in read sensor signal processing.
The present invention provides a digital recorder capable of storing digital data on a medium, a data writing mechanism capable of writing the data bits sufficiently close together on the medium to create adjacent symbol interference when read by the data access sensor, a data sensor capable of sensing the composite signal received by the interfering signals, a read sensor signal processing element capable of simultaneously demodulating and recovering the interfering signals, servo mechanisms for moving the read and write sensors over the storage medium and a system control mechanism for scheduling the motion of the read and write sensors and communicating data to and from the digital recorder. In most applications, it is desirable to minimize the power consumed by the digital recorder. Using the read mechanism of the present invention allows the disk to spin at a slower speed with slower clocking on the digital electronics used to process the signals read from the storage element.
Similar issues can be addressed in discussing optical disk systems. Intersymbol spacing must be sufficient to prevent intersymbol interference in the read sensing mechanism. Providing a read sensing mechanism able to simultaneously recover interfering optical signals would provide the benefits discussed above for magnetic disk drives in terms of greater data density on the storage medium, faster access time and reduced device power consumption.
And the same issues apply to magnetic tape systems. The spacing of adjacent tracks on the tape is also limited by intertrack interference. Providing the read sensor with means to separate closely spaced interfering signals also would provide the benefits discussed above for magnetic disk drives in terms of greater data density on the storage medium, faster access time and reduced device power consumption.
A further object is a method and apparatus for minimizing or eliminating adjacent track interference, thus leading to increased data reliability, increased track density and increased overall data capacity in tape drives. In one embodiment additional information is placed in the guard area so the guard area can be used for data that is more easily distinguished from the tracks. The guard area is defined as blank space between tracks that is ordinarily used to separate signals to prevent adjacent track interference.
In one embodiment the present invention utilizes diversity enhanced MUD by making at least one pass with one head and using the data from the multiple passes to provide better accuracy. Alternatively, there can be one pass with two heads to accomplish the same accuracy which in essence uses data from different spatial orientations.
One object of the invention is a system for reading data from a storage medium, comprising a storage surface on the storage medium having encoded data bits defined by in-track spacing and cross-track spacing, wherein the encoded data bits are stored in a plurality of data tracks. There is a means for positioning a read element over the storage surface, wherein the read element simultaneously detects the encoded data bits from several adjacent tracks. There is also a means for conditioning the encoded data bits from the read element and a means for demodulating the encoded data bits from the adjacent tracks.
An object of the invention includes an apparatus for reading data bits from a storage medium using multi-user detection, comprising a plurality of tracks wherein the data bits reside within a plurality of storage cells on the tracks. There is at least one read element simultaneously detecting a plurality of the tracks and converting the data bits into a plurality of electrical signals. A front end unit is used for processing the electrical signals and converting the electrical signals into a plurality of digital bits. A parameter estimator is coupled to the front end unit for identifying a track transfer function for the plurality of tracks. Finally, a multi-user detector is coupled to the parameter estimator and the front end unit for separating the tracks and reading the data bits.
Another object includes the storage medium selected from the group comprising floppy disks, hard disks, cubical disks, linear disks, multi-level disks, liner tapes, radial disks, compact disks, digital video disks, magneto optical disks, and rotating magnetic media. In addition, a storage medium, wherein the data bits are stored on the storage medium by a storage technology selected from the group comprising magnetic, optical, magneto optical, electrostatic, and quantum.
An additional object is the apparatus for reading data bits from a storage medium wherein the track transfer function includes envelope information of a shape, amplitude and phase of each of the plurality of data tracks. Furthermore, wherein the digital bits are represented by a Lorentzian pulse shape.
Yet another object is the apparatus for reading data bits, further comprising a guard-track spacing providing a separation between adjacent tracks. Alternatively, the plurality of data tracks can be proximate each other without a guard-track spacing. In addition, the data tracks can be multi-layered.
A further object is the apparatus for reading data bits, wherein the front end unit comprises a preamplifier, a low pass filter and an analog-to-digital converter. The apparatus can further comprise a temporary storage buffer coupled to the multi-user detector and an output multiplexor for parallel processing. A further addition is a filter unit coupled to the multi-user detector, wherein the filter unit can be a whitening matched filter bank or a matched filter bank. There can also be a sector cache coupled to the multi-user detector for buffering the data.
Another object includes the apparatus for reading data bits, wherein the multi-user detector is selected from the group comprising a maximum likelihood MUD, TurboMUD, and linear algebra based multi-user detector. The multi-user detector can also employ an algorithm selected from the group comprising an M-algorithm, T-algorithm, or MT-algorithm, based upon MAP, Log-MAP, or Max-Log MAP detectors. The apparatus for reading data bits also includes wherein the symbols use codings selected from the group comprising quadrature phase shift keying (QPSK), binary phase shift keying (BPSK), Code Division Multiple Access (CDMA), quadrature amplitude modulation (QAM), Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA) amplitude modulation (AM).
An object of the invention is a method for processing data bits of a storage medium, comprising the steps of reading a plurality of analog signals corresponding to the data bits from several adjacent tracks of the storage medium, digitizing the analog signals into digital data, generating a track transfer function of the digital data, and demodulating the digital data. An added object is the method for processing receiver signals, further comprising the step of reading at least once.
An object of the invention is an apparatus for reading and writing digital data, comprising, a storage medium wherein the digital data is represented by a plurality of storage cells on a plurality of tracks, each of the storage cells having an in-track spacing and a cross-track spacing, and each track separated by a guard-track spacing. There is a read sensor oriented to capture at least one storage cell from at least one track, with a write element oriented to write to a storage cell. A servo system is coupled to the read element, the write element and the storage medium with a system controller coupled to the servo system. A signal conditioner is coupled to the read element, wherein the signal conditioner comprises a front end unit, a parameter estimator, and a joint detector for processing the data bits from the plurality of tracks.
Further objects include the apparatus for reading and writing digital data, wherein the read element is stationary and the storage medium is moveable; the read element is moveable and the storage medium is stationary; and the read element is moveable and the storage medium is moveable.
In summary, the present invention technique applies to any storage medium where the sensor is close to a storage medium and that there is relative motion defining in-track and cross-track interference. The sensing element picks up information signals and interference and provides information processing of dense storage devices and mitigates cross-track interference. Several related applications by the common assignee are incorporated by reference herein related to parameter estimation and MUD processing.
Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein we have shown and described only a preferred embodiment of the invention, simply by way of illustration of the best mode contemplated by us on carrying out our invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention.